1. Technical Field
The present invention relates generally to a system and process for converting a light hydrocarbon gas to a heavier synthetic hydrocarbon liquid and, more particularly, to a gas conversion system and process employing a Brayton cycle in combination with an autothermal reformer and a Fischer-Tropsch reactor.
2. Background Information
A need has long existed for converting available carbonaceous materials to scarce liquid hydrocarbon fuels having preferred performance characteristics in many applications, such as internal combustion engines, jet engines and open-cycle gas turbines. Thus, for example, U.S. Pat. No. 3,986,349 teaches a process for converting solid coal to a liquid hydrocarbon fuel by gasifying the coal to a synthesis gas, hydrogenating the resulting synthesis gas, and recovering a liquid hydrocarbon fuel from the hydrogenation product. The liquid hydrocarbon fuel is used to generate power by relatively clean combustion in an open-cycle gas turbine.
Natural gas is often plentiful in regions that are uneconomical to develop because of the lack of local markets for the gas or the high cost of transporting the gas to remote markets. An alternative is to produce the natural gas and convert it in the field to a more utilitarian liquid hydrocarbon fuel or liquid chemical product for local usage or for more cost-effective transportation to remote markets. Processes for converting light hydrocarbon gases, such as natural gas, to heavier hydrocarbon liquids are generally known in the prior art. Such processes typically involve the "indirect" conversion of methane to synthetic paraffinic hydrocarbon compounds, wherein methane is first converted to a synthesis gas containing hydrogen and carbon monoxide followed by conversion of the synthesis gas to synthetic paraffinic hydrocarbon compounds via a Fischer-Tropsch reaction. The unconverted synthesis gas remaining after the Fischer-Tropsch reaction is usually catalytically reconverted to methane via a methanation reaction and recycled to the process inlet to increase the overall conversion efficiency of the process.
Conversion of methane to a synthesis gas is often performed by high-temperature steam reforming, wherein methane and steam are reacted endothermically over a catalyst contained within a plurality of externally-heated tubes mounted in a large fired furnace. Alternatively, methane is converted to a synthesis gas via partial-oxidation, wherein the methane is exothermically reacted with purified oxygen. Partial oxidation using purified oxygen requires an oxygen separation plant having substantial compression capacity and correspondingly having substantial power requirements. Production of the synthesis gas via either of the above-recited means accounts for a major portion of the total capital cost of a plant converting methane to paraffinic hydrocarbons.
Autothermal reforming is a lower cost means of converting methane to a synthesis gas. Autothermal reforming employs a combination of partial oxidation and steam reforming. The endothermic heat required for the steam reforming reaction is obtained from the exothermic partial oxidation reaction. Unlike the above-recited partial oxidation reaction, however, air is used as the source of oxygen for the partial oxidation reaction. In addition, the synthesis gas produced by autothermal reforming contains substantial quantities of nitrogen from the inlet air. Consequently, it is not possible to recycle the unconverted components contained in the process tail gas without undesirably accumulating an excess of nitrogen within the process. Production of a nitrogen-diluted synthesis gas via autothermal reforming or partial-oxidation using air followed by conversion of the synthesis gas via a Fischer-Tropsch reaction as disclosed in U.S. Pat. Nos. 2,552,308 and 2,686,195 is, nevertheless, a useful means for obtaining synthetic hydrocarbon liquid products from methane.
U.S. Pat. No. 4,833,170 discloses another example of autothermal reforming, wherein a gaseous light hydrocarbon is reacted with air in the presence of recycled carbon dioxide and steam to produce a synthesis gas. The synthesis gas is reacted in the presence of a hydrocarbon synthesis catalyst containing cobalt to form a residue gas stream and a liquid stream comprising heavier hydrocarbons and water. The heavier hydrocarbons are separated from the water and recovered as product. The residue gas is catalytically combusted with additional air to form carbon dioxide and nitrogen which are separated. At least a portion of the carbon dioxide is recycled to the autothermal reforming step.
Although prior art hydrocarbon gas conversion processes such as disclosed in U.S. Pat. No. 4,833,170 may be relatively effective for converting the light hydrocarbon gases to heavier hydrocarbon liquids, such processes have not been found to be entirely cost effective due to significant capital equipment and energy costs attributable to compression of the inlet air. The power required to compress the inlet air represents the majority of the mechanical power required to operate the process, yet much of this power is essentially lost as unrecovered pressure energy in the residue gas from the process. The inlet air requiring compression contains substantial quantities of nitrogen that remain essentially chemically inert as the nitrogen passes through the process, ultimately exiting the process in the residue gas. Furthermore, although the residue gas has a significant chemical-energy fuel value attributable to the carbon monoxide, hydrogen, methane and heavier hydrocarbon components thereof, the residue gas is very dilute, having a low heating value that renders it very difficult and costly to recover the energy of the fuel value of the residue gas with high efficiency. Thus, it is apparent that a need exists for a more cost effective hydrocarbon gas conversion process.
Accordingly, it is an object of the present invention to provide an effective process for converting a light hydrocarbon gas to a heavier synthetic hydrocarbon liquid. It is also an object of the present invention to provide an effective system of process equipment for converting a light hydrocarbon gas to a heavier synthetic hydrocarbon liquid. More particularly, it is an object of the present invention to provide such a hydrocarbon gas conversion system and process having substantially reduced power requirements. It is another object of the present invention to provide such a hydrocarbon gas conversion system and process having substantially reduced capital equipment costs. It is yet another object of the present invention to provide such a hydrocarbon gas conversion system and process emitting substantially reduced levels of contaminants to the environment. These objects and others are achieved in accordance with the invention described hereafter.